Problem: After a special medicine is introduced into a petri dish containing a bacterial culture, the number of bacteria remaining in the dish decreases rapidly. The number of bacteria decays by a factor of $\dfrac{1}{15}$ every $6.7$ minutes, and can be modeled by a function, $N$, which depends on the amount of time, $t$ (in minutes). Before the medicine was introduced, there were $90{,}000$ bacteria in the Petri dish. Write a function that models the number of bacteria $t$ minutes since the medicine was introduced. $N(t) = $
The strategy We can model the situation with an exponential function of the general form A ⋅ B f ( t ) A\cdot B\^{ f(t)}, where $A$ is the initial quantity, $B$ is a factor by which the quantity is multiplied over constant time intervals, and $f(t)$ is an expression in terms of $t$ that determines those time intervals. Let's use the given information to determine $A$, $B$, and $f(t)$. Understanding what's given We are given that the initial number of bacteria is $90{,}000$, and the number of bacteria decays by a factor of $\dfrac {1}{15}$ in $6.7$ minutes. This means that the initial quantity is $A=90{,}000$ and the factor is $B=\dfrac{1}{15}$. We need to find $f(t)$ based on the fact that the quantity is multiplied by $\dfrac{1}{15}$ every $6.7$ minutes. Finding the expression in the exponent We know that the number of bacteria is multiplied by $\dfrac{1}{15}$ every $6.7$ minutes. This means that each time $t$ increases by $6.7$, $f(t)$ increases by $1$. Therefore, $f(t)$ is a linear function whose slope is $\dfrac{1}{6.7}$. When the initial measurement is made, the number of bacteria hasn't changed. So $N(0) = 90{,}000$, which means that $f(0)=0$. [Why?] Therefore, $f(t)$ must be $\dfrac{t}{6.7}$. Summary We found that the following function models the number of bacteria $t$ minutes since the medicine was introduced. N ( t ) = 90,000 ⋅ ( 1 15 ) t 6.7 N(t)=90{,}000\cdot \left(\dfrac{1}{15}\right)\^{ \frac{t}{6.7}}